Asymmetric FET formed through use of variable pitch gate for use as logic device and test structure

ABSTRACT

Asymmetric FET devices and methods for fabrication thereof that employ a variable pitch gate are provided. In one aspect, a FET device is provided. The FET device includes a wafer; a plurality of active areas formed in the wafer; a plurality of gate stacks on the wafer, wherein at least one of the gate stacks is present over each of the active areas, and wherein the gate stacks have an irregular gate-to-gate spacing such that for at least a given one of the active areas a gate-to-gate spacing on a source side of the given active area is greater than a gate-to-gate spacing on a drain side of the given active area; spacers on opposite sides of the gate stacks; and an angled implant in the source side of the given active area.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. application Ser. No.13/434,128 filed on Mar. 29, 2012, the disclosure of which isincorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to integrated circuits, and moreparticularly, to asymmetric field-effect transistor (FET) devices andmethods for fabrication thereof that employ a variable pitch gate.

BACKGROUND OF THE INVENTION

With the continued scaling of metal-oxide semiconductor (MOS) circuits,technical challenges arise. For example, there is a point beyond whichthe gate oxide thickness cannot be reduced due to the increase inleakage current that is experienced. As a result, this limitation ongate oxide thickness can lead to short channel effects which degradeperformance of the device. Non-uniform doping profiles using angled haloor extension implants have been employed to reduce these short channeleffects.

However, as device pitch is further scaled, problems arise regardingangled halo/extension implants and the effect adjacent devices have inshielding and thus preventing proper use of this technique. For example,angled halo or extension implants are generally performed at a 20 degreeto 30 degree tilt angle. However with small gate-to-gate spacing (tightpitch) the gates from adjacent devices prevent the angled implant frombeing performed (i.e., due to shielding).

Therefore, techniques for fabricating asymmetric field-effect transistor(FET) devices at a dense pitch would be desirable.

SUMMARY OF THE INVENTION

The present invention provides asymmetric field-effect transistor (FET)devices and methods for fabrication thereof that employ a variable pitchgate. In one aspect of the invention, a method for fabricating a FETdevice is provided. The method includes the following steps. A wafer isprovided. The wafer can be a bulk semiconductor wafer or asemiconductor-on-insulator (SOI) wafer. A plurality of active areas isformed in the wafer using shallow trench isolation (STI). A plurality ofgate stacks is formed on the wafer, wherein at least one of the gatestacks is present over each of the active areas, and wherein the gatestacks have an irregular gate-to-gate spacing such that for at least agiven one of the active areas a gate-to-gate spacing on a source side ofthe given active area is greater than a gate-to-gate spacing on a drainside of the given active area. Spacers are formed on opposite sides ofthe gate stacks. An angled implant is performed into the source side ofthe given active area. The gate stacks can be formed each having a gatelength L, wherein the gate length L is configured to produce thegate-to-gate spacing on the source side of the given active area.

In another aspect of the invention, a FET device is provided. The FETdevice includes a wafer; a plurality of active areas formed in thewafer; a plurality of gate stacks on the wafer, wherein at least one ofthe gate stacks is present over each of the active areas, and whereinthe gate stacks have an irregular gate-to-gate spacing such that for atleast a given one of the active areas a gate-to-gate spacing on a sourceside of the given active area is greater than a gate-to-gate spacing ona drain side of the given active area; spacers on opposite sides of thegate stacks; and an angled implant in the source side of the givenactive area.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional diagram illustrating a starting structurefor a field-effect transistor (FET) device fabrication process, i.e., abulk semiconductor wafer, in which a plurality of active areas has beenformed according to an embodiment of the present invention;

FIG. 1B is a cross-sectional diagram illustrating an alternativestarting structure for a FET device fabrication process, i.e., asilicon-on-insulator (SOI) wafer having an active layer separated from asubstrate by a buried oxide (BOX), in which a plurality of active areashas been formed according to an embodiment of the present invention;

FIG. 2A is a cross-sectional diagram illustrating a plurality of gatestacks having been formed on the wafer with an irregular pitch in thebulk wafer configuration according to an embodiment of the presentinvention;

FIG. 2B is a cross-sectional diagram illustrating a plurality of gatestacks having been formed on the wafer with an irregular pitch in theSOI wafer configuration according to an embodiment of the presentinvention;

FIG. 3A is a cross-sectional diagram illustrating spacers having beenformed on opposite sides of the gate stacks and an angled halo/extensionimplant being performed in the bulk wafer configuration according to anembodiment of the present invention;

FIG. 3B is a cross-sectional diagram illustrating spacers having beenformed on opposite sides of the gate stacks and an angled halo/extensionimplant being performed in the SOI wafer configuration according to anembodiment of the present invention;

FIG. 4A is a cross-sectional diagram illustrating a magnified view ofFIG. 3A which further details the angled halo/extension implant processin the bulk wafer configuration according to an embodiment of thepresent invention;

FIG. 4B is a cross-sectional diagram illustrating a magnified view ofFIG. 3A which further details the angled halo/extension implant processin the SOI wafer configuration according to an embodiment of the presentinvention;

FIG. 5A is a cross-sectional diagram illustrating an alternativeembodiment wherein a plurality of gate stacks has been formed on thebulk wafer (of FIG. 1A) with a regular pitch according to an embodimentof the present invention;

FIG. 5B is a cross-sectional diagram illustrating an alternativeembodiment wherein a plurality of gate stacks has been formed on the SOIwafer (of FIG. 1B) with a regular pitch according to an embodiment ofthe present invention;

FIG. 6A is a cross-sectional diagram illustrating a mask having beenformed over the gate stacks masking those gate stacks that are to remainand leaving exposed those gate stacks that are to be removed in the bulkwafer configuration according to an embodiment of the present invention;

FIG. 6B is a cross-sectional diagram illustrating a mask having beenformed over the gate stacks masking those gate stacks that are to remainand leaving exposed those gate stacks that are to be removed in the SOIwafer configuration according to an embodiment of the present invention;

FIG. 7A is a cross-sectional diagram illustrating an etch through themask having been performed to remove the exposed gate stacks in the bulkwafer configuration according to an embodiment of the present invention;

FIG. 7B is a cross-sectional diagram illustrating an etch through themask having been performed to remove the exposed gate stacks in the SOIwafer configuration according to an embodiment of the present invention;

FIG. 8A is a cross-sectional diagram illustrating spacers having beenformed on opposite sides of the gate stacks and an angled halo/extensionimplant being performed in the bulk wafer configuration according to anembodiment of the present invention;

FIG. 8B is a cross-sectional diagram illustrating spacers having beenformed on opposite sides of the gate stacks and an angled halo/extensionimplant being performed in the SOI wafer configuration according to anembodiment of the present invention;

FIG. 9 is a cross-sectional diagram illustrating gate pitch,gate-to-gate spacing and gate length measurements according to anembodiment of the present invention; and

FIG. 10 is a cross-sectional diagram illustrating how increasing thegate length can increase the gate-to-gate spacing according to anembodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As provided above, as device pitch increases, the ability to performangled halo and/or extension implants is hampered by the shieldingeffect the tight gate-to-gate spacing has on adjacent (neighboring)devices. Advantageously, techniques are provided herein to overcome thisproblem and permit the formation of asymmetric field-effect transistor(FET) devices with a highly scaled pitch. In general, the presenttechniques are directed to increasing the gate-to-gate spacing whichthereby avoids the shielding problem. Several techniques will bedescribed herein to attain increased gate-to-gate spacing. The first twoapproaches involve employing an irregular gate pitch by irregular gatestack patterning to selectively produce a relatively larger gate pitchor by eliminating a gate stack where needed, e.g., adjacent to anasymmetrical device, thereby avoiding the shielding effect the smallergate pitch/eliminated gate stack would otherwise have on the adjacentdevice. Gate pitch is defined herein as a distance from the middle ofone gate stack to the middle of the next adjacent gate stack. In thefollowing description, the terms “gate” and “gate stack” are usedinterchangeably.

The third approach involves varying the gate length (see FIG. 9,described below), to achieve a desired (e.g., greater) gate-to-gatespacing. By using these techniques to selectively increase thegate-to-gate spacing where needed, angled halo and/or extension implantscan be easily performed without the complication of shielding byadjacent device gates.

The first approach will now be described. As highlighted above, thisapproach involves forming gate stacks having an irregular pitch producedduring the gate patterning step so as to increase gate-to-gate spacing.In general, each FET includes a source and a drain interconnected by achannel. A gate (which is typically separated from the channel by adielectric material(s), i.e., a gate dielectric) regulates current flowthrough the channel.

The process begins with a wafer such as a bulk semiconductor (silicon(Si), germanium (Ge), silicon-germanium (SiGe)) wafer or a semiconductor(Si, Ge or SiGe)— on-insulator (SOI) wafer. A SOI wafer generallyincludes a SOI layer (formed from the semiconductor material) that isseparated from a substrate (e.g., a semiconductor Si, Ge or SiGesubstrate) by a buried oxide (or BOX).

As shown in FIGS. 1A and 1B, a plurality of active areas is formed inthe wafer. FIG. 1A illustrates the instance where a bulk semiconductorwafer is employed and FIG. 1B illustrates the instance where an SOIwafer is employed. In both the bulk wafer and the SOI wafer embodiments,shallow trench isolation (STI) is used to form the active areas in thewafer. STI involves patterning trenches in the wafer and then fillingthe trenches with an insulator material, such as an oxide. See FIGS. 1Aand 1B. In the case of the SOI wafer, as shown in FIG. 1B, the STItrenches extend through the SOI layer down to the BOX in order toachieve complete isolation of the active areas.

Next, a plurality of gate stacks 202/204 is formed on the wafer. SeeFIGS. 2A and 2B, respectively. FIG. 2A (which follows from FIG. 1A)depicts the bulk wafer embodiment, whereas FIG. 2B (which follows fromFIG. 1B) depicts the SOI wafer embodiment. As shown in FIGS. 2A and 2B,the gate stacks 202/204 are formed having an irregular pitch.Specifically, a first gate pitch is employed for some regions while asecond, larger gate pitch is employed for other regions. Thus, the gatestacks formed in one or more regions of the wafer have a first gatepitch and the gate stacks formed in one or more other regions have asecond gate pitch, where the second gate pitch is larger than the firstgate pitch. Using a non-limiting example to illustrate this concept, fora given gate pitch x, a first gate pitch of 1x may be employed for someregions of the wafer, while a second (larger) gate pitch of 1.5x, 2x,3x, etc. is employed for other regions. Thus for instance, if a gatepitch x of 200 nanometers (nm) is being employed, the first gate pitchwill be 200 nm and the second gate pitch will be 400 nm. Theabove-provided gate pitch values (e.g., 1.5x, 2x, 3x, etc.) are merelyexemplary and, according to the present techniques, any gate pitchvalues may be employed as long as there is at least one region of thewafer having gate stacks with a first pitch and at least one otherregion of the wafer having gate stacks with a second pitch such that thesecond pitch is greater than the first pitch. In practice, the irregulargate pattern could be achieved (using for example, optical lithography,e-beam lithography, etc. as described above) with any two gate pitchesdesired. As will be apparent from the description below, the pitchemployed in the second (larger) gate pitch regions will depend on theangle of the tilted implant that is being employed in the asymmetricdevices.

As is apparent from FIGS. 2A and 2B, selectively employing a larger gatepitch in one or more regions of the wafer increases the gate-to-gatespacing in these regions vis-à-vis the regions of the wafer having asmaller gate pitch. See also FIG. 9, described below. FIGS. 2A and 2Bportray the non-limiting example described above wherein the gate stacksformed in one or more regions of the wafer have a (first) 1x gate pitchand the gate stacks formed in one or more other regions have a (second)2x gate pitch. This is merely an example being provided to illustratethe present techniques. As highlighted above, any irregular gate pitchvalues may be employed (e.g., 1.5x, 2x, 3x, etc.) as long as there is atleast one region of the wafer having gate stacks with a first pitch andat least one other region of the wafer having gate stacks with a secondpitch such that the second pitch is greater than the first pitch.

Accordingly, the gate stacks in this embodiment are produced having anirregular gate-to-gate-spacing, with the gate-to-gate spacing in the (inthis example the 1x gate pitch regions) being smaller than thegate-to-gate spacing in the (in this example the 2x gate pitch regions).As will be described in detail below, the present techniques are used toincrease/employ a larger gate-to-gate spacing on a source side of one ormore of the active areas. This will permit the space necessary (freefrom adjacent gate shielding) to perform an angled halo or extensionimplant into the active area(s).

By way of example only, as shown in FIGS. 2A and 2B, for a given activearea a, the pitch on a source side (in this case to the left) of theactive area a is larger (in this example 2x), whereas the pitch on adrain side (in this case to the right) of the active area a is smaller(in this example 1x), wherein for example 2x>1x. Reference to FIG. 9described below, for example, reveals that this configurationcorresponds to a greater gate-to-gate spacing on the source side of theactive area a as compared to the gate-to-gate spacing on the drain sideof the active area a. Accordingly, the increased gate-to-gate spacing onthe source side of the active area will permit the space needed toperform angled implants into the source side of the active area a(seebelow).

Gate stacks in a conventional device fabrication scheme are typicallypatterned using an optical lithography process (such asphotolithography). As is known in the art, gate stack opticallithography generally involves depositing the gate stack material(s),depositing a resist on the gate stack material(s), exposing the resist(e.g., using x-ray, ion beam, extreme ultraviolet (EUV)) through alithography mask to form a positive or negative resist with thefootprint and location of the gate stacks, etching the gate stackmaterial(s) through the resist to form the gate stacks and thenstripping the resist. However, while optical lithography is well-suited(i.e., high production yield) for regular, repeating patterns, opticallithography is (in practice) difficult to implement in situations suchas the present example where complex, irregular gate patterns aredesired due to the level of process variation that would likely result.However, an irregular pattern optical lithography mask could be createdif so desired, and is within the capabilities of one skilled in the art.Such an irregular optical lithography mask could then be implemented,according to the present techniques, along with an optical lithographyprocess to create the instant irregular gate patterns.

One exemplary alternative process to conventional optical lithographyfor creating the irregular gate pattern in this example is electron beam(e-beam) lithography. Like with optical lithography, e-beam lithographyinvolves patterning a resist, positive or negative, on the gate stackmaterial(s) with the footprint and location of the gate stacks,patterning the gate stack material(s) and then stripping the resist. Byway of example only, the gate stack materials might include a dielectricmaterial such as silicon dioxide or a high-k dielectric such as hafniumoxide, followed by a metal(s) or polysilicon. Suitable e-beam resistmaterials include, but are not limited to, poly methyl methacrylate(PMMMA). By comparison with optical lithography, e-beam lithography iswell suited for forming complex patterns. Rather than patterning theresist using a mask (as with optical lithography), e-beam lithography (amaskless process) involves emitting a beam of electrons in a patternedfashion to pattern the resist. By way of example only, direct writee-beam systems employ an e-beam spot that is moved with respect to thesubstrate being patterned to expose the substrate one pixel at a time.Accordingly, very complex and intricate patterns can be created usinge-beam lithography. Thus, according to the present techniques, anypatterning process capable of producing irregular patterned gate stacksmay be employed. To improve production yield, it may be desirable toemploy a high-speed process for creating complex patterns, such ase-beam lithography. However, optical lithography may be adapted andemployed to create the irregular gate patterns if so desired.

Further, the dimensions of the gate stacks may be chosen and/orselectively tailored so as to increase the gate-to-gate spacing. Gatelength, for example, can be selectively decreased for one or more gates,thereby increasing the gate-to-gate spacing (for a given gate pitch) forthese reduced-length gates. See FIGS. 9 and 10, described below. Thus,by way of example only, the gate-to-gate spacing achieved by patterninggates with an irregular gate pitch can be further tailored (e.g.,increased further) by selectively scaling the length of one or more ofthe gates (as illustrated, for example, in FIG. 10) so as to achieve aneven larger gate-to-gate spacing. According to an exemplary embodiment,the gates are each patterned having a common gate length L and anadditional step is employed to selectively reduce the gate length of oneor more of the gates. Following this gate length reducing step, one ormore of the gate stacks will have a smaller gate length, i.e.,L_(reduced), as compared to the gate length L of the other (unaltered)gate stacks. By way of example only, in the context of the exemplaryprocedure shown illustrated in FIGS. 2A and 2B, the length of one ormore of the gate stacks can be reduced to further increase gate-to-gatespacing. For instance, reducing the length of the gate stack (asindicated in FIGS. 2A and 2B) will further increase the gate-to-gatespacing beyond that achieved by patterning the gate stacks with a (e.g.,2x) pitch. This gate length scaling process can be performed on any ofthe gate stacks and is thus not limited to those indicated in FIGS. 2Aand 2B. This gate length scaling process may or may not result in achange in gate pitch. Namely, as described above, gate pitch is definedherein as a distance from the middle of one gate stack to the middle ofthe next adjacent gate stack. Thus, if the gate length scaling processis performed uniformly to thin both sides of the gate stack (see FIGS. 9and 10, described below), then the gate pitch may not change, but thegate-to-gate spacing will increase. According to an exemplaryembodiment, the gate length of one or more of the gate stacks is reducedby patterning a hardmask (e.g., a nitride hardmask) that i) completelycovers those gate stacks which will not be altered and ii) covers acentral portion of the gate stack(s) the length of which will bereduced. See, for example, FIG. 10, described below. Of course, theamount of the hardmask covering the gate stack(s) to be altered (and howmuch of those gate stacks remain exposed) is dependent on how muchthinning of the gate stack(s) is desired. The formation of a hardmaskmeeting these requirements is well within the capabilities of oneskilled in the art. An etch (for example a reactive ion etching (RIE)process) through the hardmask can then be used to remove gate stackmaterial from the selectively masked gate stack(s) (again see, forexample, FIG. 10, described below) to thin and thereby reduce the lengthof those etched gate stacks. Any hardmask remaining after the etch canbe removed using an etching process such as chemical mechanicalpolishing (CMP).

Following formation of the gate stacks 202/204, spacers 302/304 may beformed on opposite sides of the gate stacks 202/204. See FIGS. 3A and3B, respectively. FIG. 3A (which follows from FIG. 2A) depicts the bulkwafer embodiment, whereas FIG. 3B (which follows from FIG. 2B) depictsthe SOI wafer embodiment. According to an exemplary embodiment, spacers302/304 are formed by first blanket depositing a nitride material (e.g.,silicon nitride) onto the wafer (e.g., using chemical vapor deposition(CVD) and then patterning the spacers 302/304 using a nitride-selectivereactive ion etching (RIE).

As shown in FIGS. 3A and 3B, an angled halo or extension implant is thenperformed. Angled implants are performed to ensure that implanteddopants penetrate under the edge of the gate stack (see, for example,FIGS. 4A and 4B). As will be described in detail below, in the case ofhalo implants, the present techniques may be used to determine anoptimum angle (or strength) of the angled halo implant as per deviceperformance. According to an exemplary embodiment, the implant ispreferably performed at an angle θ of from about 20 degrees to about 30degrees into the source side of the active areas having anincreased/that employ a larger gate-to-gate spacing. Implants at more orless aggressive angles may also be employed in the same manner. Ashighlighted above, the pitch employed in the second (larger) gate pitchregions of the wafer will depend on the angle of the tilted implant thatis being employed in the asymmetric devices. Thus, a smaller, lessaggressive angle of implant will require a greater pitch (e.g., a 20degree implant will require more space and thus a greater pitch thanwould a 30 degree implant). Suitable implant dopants include, but arenot limited to, p-type dopants (for an n-channel field effect transistor(FET) device) such as boron (B), indium (In), boron fluoride (BF₂),etc., or n-type dopants for a p-channel (FET) device) such asphosphorous (P), arsenic (As), antimony (Sb), etc. FIGS. 4A and 4B showan enlarged section of FIGS. 3A and 3B, respectively, to better detailthe halo/extension implant process. The devices receiving an angled haloor extension implant are referred to herein as asymmetrical devices,while those devices not receiving an angled halo or extension implantare referred to herein as symmetrical devices.

For those devices not receiving an angled halo or extension implant(i.e., the symmetrical devices), a step may be employed (not shown) toperform a conventional vertical implant. According to an exemplaryembodiment, this vertical implant step is performed either immediatelybefore, or immediately after the angled halo or extension implant isperformed. As above, suitable implant dopants include, but are notlimited to, p-type dopants (for an n-channel FET device) such as B, In,BF₂, etc., or n-type dopants for a p-channel FET device) such as P, As,Sb, etc. Any further standard processing steps may also be performed, ifso desired, to complete the device. By way of example only, raisedsource and drain regions may be formed using conventional epitaxyprocesses, and may be doped accordingly.

As highlighted above, the present techniques are directed to increasingthe gate-to-gate spacing during FET device fabrication, so as to permitangled halo/extension implants to be performed even at scaled gatepitches. It is notable that the various techniques provided herein maybe implemented independently, or alternatively, may be combined if sodesired. By way of example only, the above-described technique involvesintentionally patterning an irregular gate pattern. Now an example willbe described wherein an irregular gate pattern is achieved by firstpatterning a regular/repeating gate pattern, and then selectivelyremoving at least one gate stack to achieve an irregular gate pattern.If so desired, the above-described irregular gate patterning techniquecould be employed, followed by a process to selectively remove a gatestack as is now described.

The starting structures, i.e., a bulk semiconductor wafer or a SOIwafer, are the same as that shown in FIGS. 1A and 1B. Thus in thisexample, as was described above, the fabrication process begins byforming a plurality of active areas in the given wafer using STI. Next,a plurality of gate stacks 502/504 is formed on the wafer. See FIGS. 5Aand 5B, respectively. FIG. 5A (which follows from FIG. 1A) depicts thebulk wafer embodiment, whereas FIG. 5B (which follows from FIG. 1B)depicts the SOI wafer embodiment. By comparison with the above-describedembodiment, in this example, as shown in FIGS. 5A and 5B, the gatestacks 502/504 are formed having a regular pitch. By way of exampleonly, for a given gate pitch x (e.g., 200 nm), the gate stacks areproduced at a pitch 1x. Further, in this case, since a regular gatepitch is being patterned (see above), optical lithography can be used topattern the gates stacks 502/504. With regular/repeated pitch, opticallithography produces a high yield with minimal process variation.

As described above, optical lithography to pattern the gate stacks(e.g., gates stacks 502/504) can involve depositing the gate stackmaterial(s), depositing a resist on the gate stack material(s), exposingthe resist (e.g., using x-ray, ion beam, EUV) through a lithography maskto form a positive or negative resist with the footprint and location ofthe gate stacks, etching the gate stack material(s) through the resistto form the gate stacks and then stripping the resist. By way of exampleonly, the gate stack materials can be blanket deposited (for exampleusing spin coating) onto the wafer as a series of layers in a stack. Thegate stack materials might include a dielectric material such as silicondioxide or a high-k dielectric such as hafnium oxide, followed by ametal(s) or polysilicon.

As will be described in detail below, the dimensions of the gate stacksmay be chosen so as to increase the gate-to-gate spacing. Gate length,for example, can be decreased, thereby increasing the gate-to-gatespacing for a given gate pitch. See description below.

Following formation of gate stacks 502/504, a (e.g., nitride) mask602/604 is formed over the gate stacks masking those gate stacks thatare to remain and leaving exposed those gate stacks that are to beremoved (see below). See FIGS. 6A and 6B, respectively. FIG. 6A (whichfollows from FIG. 5A) depicts the bulk wafer embodiment, whereas FIG. 6B(which follows from FIG. 5B) depicts the SOI wafer embodiment.

An etch through the mask 602/604 is then used to remove the exposed gatestacks. See FIGS. 7A and 7B, respectively. FIG. 7A (which follows fromFIG. 6A) depicts the bulk wafer embodiment, whereas FIG. 7B (whichfollows from FIG. 6B) depicts the SOI wafer embodiment. The particularetch chemistry used will of course depend on the particular gatematerial present. However, by way of example only, for a polysilicongate stack, a polysilicon-selective RIE may be employed, and a wet etchmay be employed to remove gate metals. Any mask 602/604 remaining afterthe etch can be removed using, e.g., CMP or grinding. Further, whileFIGS. 7A and 7B show the selective removal of one gate from between twoadjacent gate stacks, this is merely exemplary. Namely, multipleadjacent gate stacks can be removed in the same manner as described,e.g., in order to achieve a greater gate pitch in that region.

The result is a plurality of gate stacks 502/504 having an irregularpitch. Specifically, a first gate pitch is present in some regions whilea second, larger gate pitch is present in other regions. Thus, the gatestacks formed in one or more regions of the wafer have a first gatepitch and the gate stacks formed in one or more other regions have asecond gate pitch, where the second gate pitch is larger than the firstgate pitch. Using a non-limiting example to illustrate this concept, fora given gate pitch x, a first gate pitch of 1x may be present in someregions of the wafer, while a second (larger) gate pitch of 2x, 3x, etc.is employed for other regions. Thus, for instance, if a gate pitch x of200 nm is being employed, the first gate pitch will be 200 nm and thesecond gate pitch will be 400 nm. In this example, this irregular gatepitch is achieved by selectively eliminating one or more of the gatestacks 502/504. It is notable that by comparison with the embodimentprovided above where patterning is used to form an irregular gatepattern, here employing a regular gate pattern and then selectivelyeliminating one or more gate stacks will result in a gate pitch that isa multiple of the starting gate pitch (e.g., 2x, 3x, etc., see above).However, as described above and below, the techniques provided hereinmay be combined to achieve any desired gate-to-gate spacing. Thus, byway of example only, a combination of the first (irregular gate stackpatterning) embodiment with this second (selective gate elimination)embodiment and/or a combination of the gate trimming (so as to reducegate stack length) embodiment with this (selective gate elimination)embodiment can be implemented to achieve any desired combination oflarger/smaller gate pitch/gate-to-gate spacing. Thus, fractional gatepitch values can be achieved in this manner, e.g., 1.5x pitch.

The above-provided gate pitch values (e.g., 1.5x, 2x, 3x, etc.) aremerely exemplary and, according to the present techniques, any gatepitch values may be employed as long as there is at least one region ofthe wafer having gate stacks with a first pitch and at least one otherregion of the wafer having gate stacks with a second pitch such that thesecond pitch is greater than the first pitch. As will be apparent fromthe description below, the pitch employed in the second (larger) gatepitch regions will depend on the angle of the tilted implant that isbeing employed in the asymmetric devices.

As is apparent from FIGS. 7A and 7B, selectively employing a larger gatepitch (by, in this case, selectively removing a gate or gates) in one ormore regions increases the gate-to-gate spacing in these regionsvis-à-vis the regions of the wafer having a smaller gate pitch. See alsoFIG. 9, described below. FIGS. 7A and 7B portray the non-limitingexample described above wherein the gate stacks formed in one or moreregions of the wafer have a (first) 1x gate pitch and the gate stacksformed in one or more other regions have a (second) 2x gate pitch. Thisis merely an example being provided to illustrate the presenttechniques. As highlighted above, any irregular gate pitch values may beemployed (e.g., 1.5x, 2x, 3x, etc.) as long as there is at least oneregion of the wafer having gate stacks with a first pitch and at leastone other region of the wafer having gate stacks with a second pitchsuch that the second pitch is greater than the first pitch.

Accordingly, the gate stacks in this embodiment are produced having anirregular gate-to-gate-spacing, with the gate-to-gate spacing in the (inthis example the 1x gate pitch regions) being smaller than thegate-to-gate spacing in the (in this example the 2x gate pitch regions).As provided herein, the present techniques are used to increase/employ alarger gate-to-gate spacing on a source side of one or more of theactive areas. This will permit the space necessary (free from adjacentgate shielding) to perform an angled halo or extension implant into theactive area(s).

By way of example only, as shown in FIGS. 7A and 7B, for a given activearea a, the pitch on a source side (in this case to the left) of theactive area a is larger (in this example 2x), whereas the pitch on adrain side (in this case to the right) of the active area a is smaller(in this example 1x), wherein for example 2x>1x. Reference to FIG. 9described below, for example, reveals that this configurationcorresponds to a greater gate-to-gate spacing on the source side of theactive area a as compared to the gate-to-gate spacing on the drain sideof the active area a. Accordingly, the increased gate-to-gate spacing onthe source side of the active area will permit the space needed toperform angled implants into the source side of the active area a(seebelow).

As described above, the dimensions of the gate stacks may be chosenand/or selectively tailored so as to increase the gate-to-gate spacing.Gate length, for example, can be selectively decreased for one or moreof the gates, thereby increasing the gate-to-gate spacing (for a givengate pitch) for these reduced-length gates. See FIGS. 9 and 10,described below. Thus, by way of example only, the gate-to-gate spacingachieved by selectively eliminating one or more of the gate stacks canbe further tailored (e.g., increased further) by selectively scaling thelength of one or more of the gates (as illustrated, for example, in FIG.10) so as to achieve an even larger gate-to-gate spacing. According toan exemplary embodiment, the gates are each patterned having a commongate length L and (following the above-described gate eliminationprocess) an additional step is employed to selectively reduce the gatelength of one or more of the gates. Following this gate length reducingstep, one or more of the gate stacks will have a smaller gate length,i.e., L_(reduced), as compared to the gate length L of the other(unaltered) gate stacks. By way of example only, in the context of theexemplary procedure shown illustrated in FIGS. 7A and 7B, the length ofone or more of the gate stacks can be reduced to further increasegate-to-gate spacing. For instance, reducing the length of the gatestack (as indicated in FIGS. 7A and 7B) will further increase thegate-to-gate spacing beyond that achieved by patterning the gate stackswith a (e.g., 2x) pitch. This gate length scaling process can beperformed on any of the gate stacks and is thus not limited to thoseindicated in FIGS. 7A and 7B. This gate length scaling process may ormay not result in a change in gate pitch. Namely, as described above,gate pitch is defined herein as a distance from the middle of one gatestack to the middle of the next adjacent gate stack. Thus, if the gatelength scaling process is performed uniformly to thin both sides of thegate stack (see FIGS. 9 and 10, described below), then the gate pitchmay not change, but the gate-to-gate spacing will increase. According toan exemplary embodiment, the gate length of one or more of the gatestacks is reduced by patterning a hardmask (e.g., a nitride hardmask)that i) completely covers those gate stacks which will not be alteredand ii) covers a central portion of the gate stack(s) the length ofwhich will be reduced. See, for example, FIG. 10, described below. Ofcourse, the amount of the hardmask covering the gate stack(s) to bealtered (and how much of those gate stacks remain exposed) is dependenton how much thinning of the gate stack(s) is desired. The formation of ahardmask meeting these requirements is well within the capabilities ofone skilled in the art. An etch (for example a RIE process) through thehardmask can then be used to remove gate stack material from theselectively masked gate stack(s) (again see, for example, FIG. 10,described below) to thin and thereby reduce the length of those etchedgate stacks. Any hardmask remaining after the etch can be removed usingan etching process such as CMP.

Following formation and selective removal of one or more of the gatestacks 502/504, spacers 802/804 may be formed on opposite sides of thegate stacks 502/504. See FIGS. 8A and 8B, respectively. FIG. 8A (whichfollows from FIG. 7A) depicts the bulk wafer embodiment, whereas FIG. 8B(which follows from FIG. 7B) depicts the SOI wafer embodiment. Accordingto an exemplary embodiment, spacers 802/804 are formed by first blanketdepositing a nitride material (e.g., silicon nitride) onto the wafer(e.g., using CVD and then patterning the spacers 802/804 using anitride-selective RIE.

As shown in FIGS. 8A and 8B, an angled halo or extension implant is thenperformed. As will be described in detail below, in the case of haloimplants, the present techniques may be used to determine an optimumangle (or strength) of the angled halo implant as per deviceperformance. The implant is preferably performed at an angle θ of fromabout 20 degrees to about 30 degrees into the source side of the activeareas having an increased/that employ a larger gate-to-gate spacing.Implants at more or less aggressive angles may also be employed in thesame manner. As highlighted above, the pitch employed in the second(larger) gate pitch regions of the wafer will depend on the angle of thetilted implant that is being employed in the asymmetric devices. Thus, asmaller, less aggressive angle of implant will require a greater pitch(e.g., a 20 degree implant will require more space and thus a greaterpitch than would a 30 degree implant). Suitable implant dopants include,but are not limited to, p-type dopants (for an n-channel FET device)such as B, In, BF₂, etc., or n-type dopants for a p-channel (FET)device) such as P, As, Sb, etc. A detailed view of the angledhalo/extension implant in a bulk/SOI wafer is shown in FIGS. 4A and 4B,described above. The devices receiving an angled halo or extensionimplant are referred to herein as asymmetrical devices, while thosedevices not receiving an angled halo or extension implant are referredto herein as symmetrical devices.

For those devices not receiving an angled halo or extension implant(i.e., the symmetrical devices), a step may be employed (not shown) toperform a conventional vertical implant. According to an exemplaryembodiment, this vertical implant step is performed either immediatelybefore, or immediately after the angled halo or extension implant isperformed. As above, suitable implant dopants include, but are notlimited to, p-type dopants (for an n-channel FET device) such as B, In,BF₂, etc., or n-type dopants for a p-channel FET device) such as P, As,Sb, etc. Any further standard processing steps may also be performed, ifso desired, to complete the device. By way of example only, raisedsource and drain regions may be formed using conventional epitaxyprocesses, and may be doped accordingly.

FIG. 9 shows how gate pitch and gate-to-gate spacing are measured. Ashighlighted above, gate pitch is measured as a distance from the middleof one gate to the middle of the next, adjacent gate. This measurementis shown in FIG. 9 as gate pitch x. The above-described techniques weredirected to (selectively) increasing the gate pitch (e.g., from 1x to2x) for certain regions so as to increase the gate-to-gate spacing. Itis apparent from FIG. 9 how increasing the gate pitch would serve toincrease the gate-to-gate spacing.

FIG. 9 also illustrates the gate length measurement, which introduces athird exemplary embodiment for achieving increased gate-to-gate spacing.This technique involves, for a given gate pitch, decreasing the gatelength to achieve increased gate-to-gate spacing. Compare, for example,FIG. 9 with FIG. 10.

In FIG. 10, the gate length of one of the gate stacks shown (in thisexample the gate stack on the left) has been decreased. As a result, thegate-to-gate spacing has been increased. The gate pitch, however, hasnot changed since gate pitch is measured from the middle or center ofthe gate. As further shown in FIG. 10, the hardmask used to cover thethinned gate stack(s) covers only a central portion of the respectivegate stack(s) (the size of the mask depending on how much the gatestack(s) is to be thinned), while the hardmask over the gate stack(s)which will not be altered completely covers these gate stacks so as toprotect them during the subsequent etch. As FIG. 10 indicates, thehardmask covering the unaltered gate stacks can be blanket deposited soas to cover multiple gate stacks, if so desired.

The details of masking and selectively thinning one or more of the gatestacks so as to reduce the length of the thinned gate stack(s) wasdescribed in detail above. This step of decreasing the gate length (soas to increase the gate-to-gate spacing) may be performed in conjunctionwith either, or both, of the embodiments (e.g., irregular gate stackpatterning/removing a gate stack) described above in order to furtherincrease the gate-to-gate spacing. However, decreasing the gate lengthmay be implemented in accordance with any other device fabricationprocess in order to achieve a greater gate-to-gate spacing, and thus isnot limited to the processes described above.

The FET devices fabricated according to the present techniques may beemployed in conventional logic circuits, such as NAND gates, AND, OR andNOR gates. In addition, the present FET devices may be used as teststructures for evaluating the impact of halo shadowing in planar devicesin that the device may be measured twice (once in one source/drainconfiguration and a second time with source and drain nodes swapped). Byexamining the asymmetry in the short channel behavior in bothmeasurements, the relative strengths of the asymmetric halos can bedetermined. By way of example only, any of the above-describedtechniques can be used to change the gate-to-gate spacing of one or moreof the FET devices. The greater the gate-to-gate spacing, the less theshadowing effect (from the adjacent device) and thus the more asymmetric(greater angle of tilted implant) can be employed. Halo strength is anindicator of how asymmetric or at how much of an angle the tiltedimplant is performed. Thus, any one of the above-described fabricationtechniques, alone or in combination (see above), can be used to producea variety of devices with varying gate-to-gate spacing. Going from asmaller gate-to-gate pitch to a larger gate-to-gate pitch, anincreasingly greater angle of implant can be used (larger asymmetry).

It is notable that device performance (measured for example using a ringoscillator as explained below), may be increased by using a stronger,more asymmetric implant (based on a greater gate-to-gate spacing). Thereare however tradeoffs. Namely, the ring oscillator speed is related tocapacitance (C), voltage (V) and current (I) as follows:CV/I.By using a more asymmetric implant, the capacitance increases, but sodoes the resistance (and resistance affects current (I)). Thus, there islikely an optimal halo strength (amount of implant asymmetry, i.e.,resulting from a certain amount of gate-to-gate spacing as produced bythe present techniques) that has a favorable amount of capacitance aswell an acceptable amount of resistance. This optimal point can bedetermined using the present techniques.

Namely, FET devices can be produced using the above-described processeshaving various different gate-to-gate spacings which permit differentstrength asymmetric implants (for example, ranging from asymmetrical/vertical implant to an aggressive 30 degree angle tiltedimplant, and values in between) to be performed. The resulting devicescan then be tested using a ring oscillator circuit. As is known in theart, a ring oscillator contains a plurality of inverters connected in aloop. Ring oscillators are commonly employed as test circuits. The speedof the ring oscillator circuit will determine the performance of thedevices, and from that data, the optimal device configuration (based inthis case on gate-to-gate spacing and resultant angle of implant) can bedetermined. It is assumed that oscillator speed will increase withincreased implant angle to a certain point, beyond which, increasing theangle of implant will decrease the oscillator speed due to increasedresistance.

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope of the invention.

What is claimed is:
 1. A FET device, comprising: a wafer; active areasformed in the wafer; gate stacks on the wafer present over one or moreof the active areas, wherein the gate stacks have an irregulargate-to-gate spacing in that the gate-to-gate spacing between a giventwo of the gate stacks next to one another on the wafer is selectivelyconfigured to be greater than the gate-to-gate spacing of the gatestacks on the wafer to either side of the given two gate stacks, whereinby way of the irregular gate-to-gate spacing: i) for at least a givenone of the active areas in which an asymmetrical transistor is present agate-to-gate spacing on a source side of the given active area isgreater than a gate-to-gate spacing on a drain side of the given activearea, and ii) for at least another given one of the active areas inwhich a symmetrical transistor is present a gate-to-gate spacing on asource side of the other given active area is equal to a gate-to-gatespacing on a drain side of the other given active area; spacers onopposite sides of the gate stacks; and an angled implant in the sourceside of the given active area, wherein the gate-to-gate spacing employedon the source side of the given active area is dependent on an angle ofthe angled implant in the source side of the given active area in thatthe gate-to-gate spacing on the source side of the given active area ischosen such that none of the gate stacks on the wafer shield the angledimplant in the source side of the given active area, and wherein thegate stacks on the wafer all have a same height, and wherein a greatergate-to-gate spacing on the source side of the given active area isemployed to permit a greater asymmetry of the angled implant in thesource side of the given active area.
 2. The FET device of claim 1,wherein the wafer comprises a bulk silicon, germanium, orsilicon-germanium wafer.
 3. The FET device of claim 1, wherein the wafercomprises a semiconductor-on-insulator (SOI) wafer having a SOI layerseparated from a substrate by a buried oxide (BOX).
 4. The FET device ofclaim 3, wherein the SOI layer comprises silicon, germanium, or silicongermanium.
 5. The FET device of claim 1, wherein the spacers comprise anitride material.
 6. The FET device of claim 5, wherein the nitridematerial is silicon nitride.
 7. The FET device of claim 1, wherein theangled implant comprises an angled halo implant or an angled extensionimplant.
 8. The FET device of claim 1, wherein the gate stacks have anirregular gate pitch.
 9. The FET device of claim 8, wherein theirregular gate pitch comprises at least a first 1x gate pitch and atleast a second 2x gate pitch such that the second gate pitch is 2xgreater than the first gate pitch.
 10. The FET device of claim 1,wherein at least one of the gate stacks has a decreased gate length thatis less than a gate length of one or more other of the gate stacks, andwherein the at least one gate stack that has the decreased gate lengthis adjacent to the source side of the given active area.
 11. The FETdevice of claim 1, wherein the gate stacks comprise at least one metal.12. The FET device of claim 1, wherein the gate stacks comprisepolysilicon.
 13. The FET device of claim 1, wherein the gate stackscomprise a dielectric material.
 14. The FET device of claim 1, whereinthe angled implant comprises a p-type dopant.
 15. The FET device ofclaim 14, wherein the p-type dopant comprises boron, indium or boronfluoride.
 16. The FET device of claim 1, wherein the angled implantcomprises an n-type dopant.
 17. The FET device of claim 16, wherein then-type dopant comprises phosphorous, arsenic or antimony.
 18. The FETdevice of claim 9, wherein one or more of the first gate pitch and thesecond gate pitch comprises a fractional gate pitch.
 19. The FET ofclaim 10, wherein a pitch of the gate stacks is unaffected by thedecreased gate length.
 20. The method of claim 1, wherein the gatestacks have an irregular gate pitch in which all but a select two of thegate stacks on the wafer have a 1x gate pitch, and wherein the selecttwo gate stacks have a 2x gate pitch.